1. Field of the Invention
Embodiments of the present invention relate generally to drilling of a wellbore and, more particularly, to drilling of a wellbore using casing as a drilling string. More particularly still, embodiments of the present invention relate generally to removal of a drilling structure at the end of a casing.
2. Description of the Related Art
The drilling of wells to recover hydrocarbons from subsurface formations is typically accomplished by directing a rotatable drilling element, such as a drill bit, into the earth on the end of tubing known as a “drill string” through which drilling mud is directed to cool and clean the drilling face of the drill bit and remove drilled material or cuttings from the wellbore as it is drilled. A typical well may have wellbore sections having different sizes. After a section of wellbore has been drilled or bored to its desired depth and location, the wellbore is typically cased, i.e., metal tubing is located along the length of the wellbore and cemented in place to isolate the wellbore from the surrounding earth, to prevent the formation from caving into the wellbore, and to isolate the earth formations from one another. After that, the next smaller size wellbore will be drilled to a deeper depth and then cased. The last casing is then perforated at specific locations where hydrocarbons are expected to be found, to enable their recovery through the wellbore.
It is known to use casing as the drill string, and, when drilling is completed to a desired depth, to cement the casing in place and thereby eliminate the need to remove the drill string from the wellbore. Thereafter, a subsequent drill string is used to drill through the prior drill string and extend the wellbore further into the earth.
However, because the prior drill string is not removed, equipment such as the drill bit, also referred to as the drill shoe, in the prior drill string may present drill out issues to the subsequent drill string. Thus, the drill shoe of the prior drill string should be eliminated as an obstacle, without pulling the prior drill string from the wellbore.
One method used to facilitate removal of the drill shoe is to manufacture the drill shoe using a drillable material. Use of drillable material, however, limits the loading that can be placed on the drill shoe during drilling, which may reduce the efficiency of drilling with the drillable drill shoe. A typical “drillable” drill shoe configuration includes a relatively soft metal, such as aluminum, with relatively hard inserts of materials such as synthetic diamond located thereon to serve as the cutting material. The hard cutters of the drill shoe also pose a concern because the hard cutters may increase wear and physical damage to the subsequent drill shoe or drill bit being used to drill through the previous drill shoe, thereby reducing the life of the subsequent drill bit and the depth of formation it can penetrate before it no longer drills effectively.
There is a need, therefore, for a method of removing the cutting structure on a casing drill string to reduce the potential for damage to the drill out tool. There is also a need for a method to remove the cutting structure on a drill shoe located at the end of a casing after the drill shoe has reached a target depth.